Void accumulation under contact vias is frequently observed inside an operating semiconductor device. This phenomena can cause the circuit to open as soon as the void size covers the whole via contact area. This failure results from a current density divergence at the contact area and is attributed to a phenomenon of “electromigration” (EM) failure inside the conductive interconnects. More specifically, this phenomena occurs when the conducting material such as, for example, aluminum or copper, migrates or moves away from the contact to form a void between the contact and metal line when a high density current is passed through the circuit. Thus, as the feature size is scaled, the EM problem becomes worse and the accumulation time before an open circuit occurs becomes shorter.
By way of one illustrative example, referring to a top view of FIG. 1, in the case that the contact via and the underneath metal line have comparable CD size, the diffusion barrier around the sidewall of the via will contact the diffusion barrier on the sidewall of the underneath metal. This barrier-to-barrier contact, i.e., barrier redundancy, offers an electrical path when EM failure occurs thus avoiding a sudden opening of the circuit. However, this barrier redundancy feature cannot be reached through a regular process when the contact via size is smaller than the underneath metal line, as shown in a top view of FIG. 2. In this latter case, the circuit will open as soon as the EM failure occurs since there is no redundancy circuitry, i.e., no redundant contact. FIG. 3 shows such a failure of the circuit due to EM failure.
Methods to improve EM resistance have been proposed in many instances. These proposals include, for example, forming an inter metallic region beneath and adjacent a conductive plug through reaction of barrier material with the underneath metal line during a thermal treatment. Additional proposals include forming an electromigration resisting layer through reaction of a correction inhibitor with the underneath metal line during a thermal treatment, as well as forming a copper plug as a reservoir for improving electromigration resistance. Another improvement in this area includes forming a Cu—Zn alloy along the surface of copper interconnects for improving electromigration resistance.